Cruz Chad D, Yuan Jennifer, Climent Clàudia, Tierce Nathan T, Christensen Peter R, Chronister Eric L, Casanova David, Wolf Michael O, Bardeen Christopher J
Department of Chemistry , University of California Riverside , 501 Big Springs Road, Riverside , California 92521 , USA . Email:
Department of Chemistry , University of British Columbia , 2036 Main Mall , Vancouver , BC , Canada V6T 1Z1.
Chem Sci. 2019 Jun 17;10(32):7561-7573. doi: 10.1039/c8sc05598j. eCollection 2019 Aug 28.
Covalently tethered bichromophores provide an ideal proving ground to develop strategies for controlling excited state behavior in chromophore assemblies. In this work, optical spectroscopy and electronic structure theory are combined to demonstrate that the oxidation state of a sulfur linker between anthracene chromophores gives control over not only the photophysics but also the photochemistry of the molecules. Altering the oxidation state of the sulfur linker does not change the geometry between chromophores, allowing electronic effects between chromophores to be isolated. Previously, we showed that excitonic states in sulfur-bridged terthiophene dimers were modulated by electronic screening of the sulfur lone pairs, but that the sulfur orbitals were not directly involved in these states. In the bridged anthracene dimers that are the subject of the current paper, the atomic orbitals of the unoxidized S linker can actively mix with the anthracene molecular orbitals to form new electronic states with enhanced charge transfer character, different excitonic coupling, and rapid (sub-nanosecond) intersystem crossing that depends on solvent polarity. However, the fully oxidized SO bridge restores purely through-space electronic coupling between anthracene chromophores and inhibits intersystem crossing. Photoexcitation leads to either internal conversion on a sub-20 picosecond timescale, or to the creation of a long-lived emissive state that is the likely precursor of the intramolecular [4 + 4] photodimerization. These results illustrate how chemical modification of a single atom in the covalent bridge can dramatically alter not only the photophysics but also the photochemistry of molecules.
共价连接的双发色团为开发控制发色团组装体中激发态行为的策略提供了理想的试验场。在这项工作中，结合光谱学和电子结构理论证明，蒽发色团之间硫连接基的氧化态不仅能控制分子的光物理性质，还能控制其光化学性质。改变硫连接基的氧化态不会改变发色团之间的几何结构，从而可以分离出发色团之间的电子效应。此前，我们表明硫桥连的对三联苯二聚体中的激子态是通过硫孤对电子的电子屏蔽来调节的，但硫轨道并未直接参与这些态。在本文所研究的桥连蒽二聚体中，未氧化的S连接基的原子轨道可以与蒽分子轨道有效混合，形成具有增强电荷转移特性、不同激子耦合以及取决于溶剂极性的快速（亚纳秒）系间窜越的新电子态。然而，完全氧化的SO桥恢复了蒽发色团之间纯粹的空间电子耦合，并抑制了系间窜越。光激发导致在亚20皮秒时间尺度上的内转换，或者产生一种长寿命的发射态，这可能是分子内[4 + 4]光二聚化的前体。这些结果说明了共价桥中单个原子的化学修饰如何不仅能显著改变分子的光物理性质，还能改变其光化学性质。
